1. Field of the Invention
The present invention relates to a television signal transmitter used for a CATV system or the like.
2. Description of the Related Art
A television signal transmitter has the function of converting the frequency of a television intermediate frequency signal to the frequency of a channel to which the signal is transmitted. As shown in FIG. 4, a conventional television signal transmitter has an intermediate frequency (IF) amplifier 51, a mixer 52, a local oscillator 53, a radio-frequency (RF) band-pass filter 54, a radio-frequency (RF) amplifier 55, a radio-frequency (RF) relay 56, and the like.
A television intermediate frequency (IF) signal outputted from a modulator (not shown) is supplied to the IF amplifier 51. A video intermediate frequency and a sound intermediate frequency of the IF signal are, for example, 45.75 MHz and 41.25 MHz, respectively, according to U.S. specifications. The IF signal amplified by the IF amplifier 51 is supplied to the mixer 52.
A local oscillation signal outputted from the local oscillator 53 is supplied to the mixer 52. The local oscillator 53 takes the form of a voltage-controlled oscillator having therein a varactor diode, and its oscillation frequency is changed by a control voltage applied to the varactor diode. The IF signal is frequency-mixed with the local oscillation signal and frequency-converted to a television signal (RF signal) of a channel through which the signal is to be transmitted. The frequency of the RF signal lies in the range from 55 MHz to 860 MHz, and the frequency of the local oscillation signal is set in correspondence with the frequency of the RF signal. The local oscillation frequency is higher than the frequency of the RF signal only by the frequency of the IF signal.
The RF signal is supplied to the RF amplifier 55 via the RF band-pass filter 54. The RF band-pass filter 54 is constructed by three band-pass filters 54a to 54c each taking the form of a double-tuned circuit having a varactor diode. The RF band-pass filter 54 is controlled so as to be tuned in the frequency band of the RF signal. The RF signal passed through the RF band-pass filter 54 is amplified by the RF amplifier 55, outputted via the RF relay 56, and supplied to a distributer, a mixer, and the like (not shown) at the next stage.
Data for setting the local oscillation frequency of the local oscillator 53 and the tuning frequency of the band-pass filter 54 is stored in a memory (ROM) 57. Data read by an MPU 58 is converted by a D/A converter 59 to an analog voltage which is applied to the varactor diode in the local oscillator 53 and the varactor diode in the band-pass filter 54.
In the above configuration, for example, when changing the frequency of the RF signal and/or the degree of modulation of the IF signal, there is the possibility that an unwanted signal generated by the change is outputted and negatively impacts other devices. Consequently, it is necessary to prevent the generation of these unwanted signals. By operating the RF relay 56 at the time of such a change so as to be switched off, the RF signal is not outputted.
When the RF relay 56 is switched off, however, since the circuit is disconnected, the impedance of the circuit is not matched to that at the output terminal of the RF amplifier 55. Problems such as unstable operation in the amplifier due to reflection and occurrence of a loss at an output terminal of the amplifier due to an impedance mismatch occurs.